Photographic technology has been in rapid development in recent decades. The progresses are mainly reflected in: (1) digital technology is used in cameras, and (2) the requirement of large field of view on cameras brings wide-angle and ultra wide-angle lens into reality. However, to successfully apply digital technology in cameras, the Moire fringes caused by using CCD or CMOS image sensors and the thermal fluctuation caused by infrared light need to be suppressed. Thus, an infrared cut-off filter needs to be installed before the CCD or CMOS in the digital camera.
As to the wide-angle lens, the general requirement for a good quality camera is a field of view greater than 60°. That is, the optical lens not only needs to capture images with the normal incident light, but also needs to capture images with oblique incident light at an angle as large as 30°. However, wave optics theory has proved: the oblique incident light can produce polarization and lead to change in the refractive index and reflectivity of the optical lens. In general, if the incident light angle changes within 5°, the change in the refractive index and reflectivity may be small. But if the incident light angle changes beyond 5°, the change in the refractive index and reflectivity may be dramatic. If the field of view of the camera is 60°, then the change in the refractive index or reflectivity may be more than 35%, or even reaches 50%˜85%, which can cause change in the photographic image clarity and serious distortion on the color of the image.
In order to reduce the changes on the refractive index of the lens caused by changes of the incidence angles, two mechanisms are currently used. One is to use blue glass and resin to make the optical lens. However, the blue glass is often costly and has few standard substrate thickness; the blue glass substrate is often in short supply; and the resin coating process is generally more complex, etc.
The other mechanism is to place an infrared (IR) film on the surface of the lens. The IR film is formed by stacking standard film layers, which may be easy to make. However, when lights enter at different incidence angles, the refractive index at 600-700 nm red wavelength range also changes. The low image clarity and uneven color can still exist, and only low-resolution optical imaging systems still use this type of IR film.
The disclosed methods and systems are directed to solve one or more problems set forth above and other problems.